1. Field of the Invention
An apparatus for rapidly applying a polymer coating to a variety of surfaces through plasma flame spraying is provided, enabling the user to work in relative safety and achieve a smooth, uniform coating. The apparatus hereof is particularly concerned with a hand-held device for plasma flame spraying a variety of polymers whereby a surface such as wood, fibrous glass reinforced synthetic resin, or even cardboard may be sprayed in close proximity to the front of the barrel of the apparatus without damaging the surface or exposing the operator to potentially toxic fumes resulting from the melted polymer. In its preferred method aspects, the invention involves applying a protective coating to a surface such as a boat hull by providing a electric arc, directing a gas stream through the arc thereby heating the gas stream, injecting a powdered polymer into the gas stream at a location downstream from the arc so as to melt the powder without overheating the same, and then applying the melted mixture to the surface to be coated.
2. Description of the Prior Art
Plasma flame spraying has proven to be a highly efficient and effective method of applying heat fusible materials to a variety of heat resistant surfaces. Plasma is an extremely hot substance consisting of free electrons, positive ions, atoms and molecules. Although it conducts electricity, it is electrically neutral. Plasma is usually generated at temperatures in the vicinity of 15,000 degrees Centigrade by the passage of a gas through an electric arc. In typical plasma spraying systems, a selected inert gas, such as argon or nitrogen, flows between an anode and a cathode. An electrical arc is generated between the anode and cathode, both heating and propelling a heat fusible material carried with the gas. The movement of the gas beween the anode and cathode effectively lengthens the path of the arc, causing more energy to be delivered to the arc. The plasma may issue from the nozzle at subsonic to Mach II speeds, with a flame of intense brightness and heat resembling an open oxy-acetylene flame.
It may be readily appreciated that the intense heat associated with the plasma stream and the rapid flow of the plasma through the gun presents a highly efficient means of melting a heat fusible material and spraying it on a target surface. The plasma flame spray guns previously developed have been principally designed to apply powdered ceramics or metals which have high melting temperatures. These materials are typically injected at or near the arc to achieve the instantaneous melting required when the plasma stream is flowing at sonic or near sonic speeds. Despite the intense heat generated at the arc, the temperature of the plasma gas stream drops rapidly across the intervening distance between the electrode and the target surface. This drop is a function of gas enthalpy, energy absorption by the powdered material, and work distance.
It has become increasingly popular to attempt to apply synthetic polymers by the plasma flame spray method. Flame sprayed polymer powders are lighter in mass and have a much lower melting point than ceramics or metals. As a result, the high temperatures of the arc tend to "burn" the polymers rendering the resulting coating unsuitable. Various plasma spray devices have been developed for use with polymers, such as that of U.S. Pat. No. 3,676,638, which discloses a nozzle whereby powder is fed into the plasma gas stream downstream from the arc. These prior plasma spray devices have been limited in the rate of application due to the low arc power settings necessary to avoid "burning" the polymer, and have had a tendency to produce a somewhat uneven coating with splattering and dangerous and inefficient overspray.
Nonetheless, the durability and density of plasma sprayed polymer coatings have produced a demand for devices which can effectively apply these coatings. In contrast to painted polymer coatings, which require a great deal of surface preparation and wear rapidly, plasma flame sprayed polymer coatings provide a wear-resistant coating of high density with a high bond strength generated as a result of the high velocity impact of the molten composition onto the target surface. In addition, only a nominal amount of grit blasting to slightly roughen the surface and remove any surface contamination is necessary to prepare the surface for plasma flame spraying. However, certain polymer compositions have heretofore been difficult to use in hand held operation because of the toxic fumes released by the molten polymer in the plasma stream. Further, prior apparatus made it difficult to prevent the plasma gas stream from scorching the surface during application. The high heat of the plasma stream in close proximity to the user also posed a safety harzard in the event a plasma gas stream were to be inadvertently directed at the user or another person. Because of the high heat generated by the plasma gas stream, the target surface remained hot after the deposit of the coating, resulting in additional release of toxic fumes into the environment.